Soft and Isotropic Phonons in PrFeAsO_{1-y}

TL;DR

This study investigates phonons in PrFeAsO_{1-y} using high-resolution x-ray scattering and ab initio calculations, revealing surprisingly isotropic phonon behavior and highlighting the limitations of current models in capturing magnetic effects.

Contribution

It demonstrates that including magnetism in calculations improves agreement but still fails to fully match experimental phonon spectra, suggesting the need for refined models.

Findings

Phonons are surprisingly isotropic despite anisotropic magnetic properties.

Including magnetism in calculations improves but does not fully match experimental data.

Modified calculations with bond clipping or force constant softening yield better agreement.

Abstract

Phonons in single crystals of PrFeAsO_{1-y} are investigated using high-resolution inelastic x-ray scattering and ab initio pseudopotential calculations. Extensive measurements of several samples at temperatures spanning the magnetic ordering temperature and the superconducting transition temperature show that there are some changes in phonon spectra with temperature and/or doping. We compare our measurements with several ab initio pseudopotential models (nonmagnetic tetragonal, oxygen-deficient O_{7/8} supercell, magnetic orthorhombic, and magnetic tetragonal) and find that the experimentally observed changes are much smaller than the differences between the experimental data and the calculations. Agreement is improved if magnetism is included in the calculations via the local spin density approximation, as the Fe atomic motions parallel to the ferromagnetic ordering direction are softened. However, the antiferromagnetically polarized modes remain hard, and in disagreement with the experimental data. In fact, given the increasing evidence for anisotropy in the iron pnictide materials, the phonon response is surprisingly isotropic. We consider several modifications of the ab initio calculations to improve the agreement with the experimental data. Improved agreement is found by setting the matrix to zero (clipping the bond) between nearest-neighbor antiferromagnetically aligned Fe atoms in the magnetic calculation, or by softening only the in-plane nearest-neighbor Fe-As force constant in the nonmagnetic calculation. We discuss these results in the context of other measurements, especially of phonons, for several FeAs systems. Fluctuating magnetism may be a partial explanation for the failure of the calculations, but seems incomplete in the face of the similarity of the measured phonon response in all the systems investigated here including those known to have static magnetism.